X-ray imaging plays an important role in daily life. Imaging-type x-ray detectors are essential for applications ranging from medical x rays to astronomy to airport security.
Generally, imaging-type detectors for x-ray applications use either chemical detectors (e.g., film or photoresist) or electronic detectors (e.g., charge-coupled device array (CCD) or imaging plate). Their characteristics are described below
Chemical-based detectors are the traditional x-ray imaging detectors. Films were first used by Röntgen to record x-ray exposures and continue to play a dominate role in medical imaging. The photoresist detectors provide extremely high imaging resolution and are used routinely in lithography applications.
The chemical detectors, however, suffer from the properties that they are single-use only and require post-exposure chemical processing to reveal or develop the image.
X-ray film detectors are widely used in medical imaging. Like photographic films, they use light-sensitive silver-halides to record the x-ray exposure and subsequent chemical development is used to reveal the latent image. The resolution of these photographic films is limited by the grain size to about 10 micrometers (μm). High-sensitivity films have coarser grains of 50–100 μm. The quantum detection efficiency of film is generally less than 5%.
Photoresists are typically thin sheets of an organic polymer. In the case of positive resists, the molecular bonds in the polymer are broken by the x-ray exposure, and the broken chains are removed by chemical development. This reveals the latent image. In the case of negative resists, the radiation exposure renders the polymer insoluble. The photoresists typically have extremely high resolution of a few nanometers, resolution being limited primarily by secondary electron exposure.
Most electronic imaging detectors are based on a CCD camera or an imaging plate. They overcome many problems associated with chemical detectors including the single-use and the requirement for post-exposure processing. Images recorded by an electronic detector can be read-out immediately after exposure.
These electronic imaging detectors, however, can suffer from radiation damage to the gate circuits. But if the radiation damage can be controlled, the detectors they typically have long service lifetimes. Imaging plates are generally used for large area and coarse resolution imaging applications.
With direct imaging CCD detectors, the CCD chip is exposed directly by the radiation to create electron-hole pairs. This system typically has very high conversion efficiency since no secondary magnification is used. There is also no loss of sampling rate due to re-sampling at a secondary magnification stage. The Nyquist-limited spatial resolution of the direct detection method is twice the pixel size. Since the pixel sizes of most CCD chips are typically 6–20 μm, the detection resolution is limited to about 10 μm. The number of electron-hole pairs created by an absorbed photon is approximately the photon energy divided by the gap energy. Hence, a hard x-ray photon with tens of kilo-electron-Volts (keV) energy can create thousands of electron-hole pairs. Since the full-well capacity of a CCD pixel is typically between 10,000 to 100,000 electrons. The dynamic range is therefore limited to about 100 for hard x rays. X-ray radiation can also damage the CCD chip, reducing the lifetime to less than 10,000 exposures.
Another configuration uses scintillated detectors with fiber-optic taper coupling. With this system, a fiber optic taper is typically bonded to the CCD chip on one side and coated with scintillation material on the other side. The system provides moderate magnification, commonly less than 10 times, with a few micrometer resolutions. The conversion efficiency is usually quite high, typically 70–80%. Radiation can be prevented from reaching the CCD directly, and thus the CCD does not suffer from radiation damage. These systems, however, suffer from distortion from the taper, which increases with the magnification.
Scintillated detector systems with photographic lens coupling use a scintillator screen to convert the x-ray photons into visible light photons. The image formed on the screen is then imaged to the CCD chip with a photographic lens. The conversion efficiency of these systems is not very high, however, since the numerical apertures of photographic lenses are generally low. Further, the resolution is typically lower than five micrometers. On the other hand, imaging performance is typically good due to the generally high imaging quality of commercial photographic lenses, and a large array of photographic lenses are commercially available to provide wide range of magnification and resolution capabilities.
Scintillated detectors with microscope objective lens provide the highest resolution, being better than a micrometer in the best cases. Among the electronic detector systems, they provide the highest resolution due to the use of the microscope objective. And because of the high resolution, only grainless single crystals are suitable scintillator material.
Moreover, the throughput can be very high with the use of high-numerical aperture objectives. These systems also have the smallest field of view due to the limitations of the objective. As in the case of photographic lenses, a large array of objectives with various magnification and numerical apertures are also commercially available to provide wide range of magnification and resolution capabilities.